POWER October 2016 - 49

GAS-FIRED POWER
2. Ice attack. The blades in this turbine compressor were damaged when water from a
leaky wash system froze in cold weather and the ice broke free and entered the turbine. Source:
POWER
3. Hot take. Hot corrosion results from
contamination of the turbine blades with salts
and other substances while at high operating
temperatures. Courtesy: Sulzer
self can cause additional fouling, erosion, and
corrosion if not conducted properly. For these
and other reasons, the use of demineralized or
better quality water is recommended for any
water that enters the turbine. The use of city
water or surface water, even temporarily, will
eventually result in problems.
Much the same concerns apply to fuel supto
keep in mind is that such debris need not be
especially large. Turbines have been gradually
destroyed by small pieces of components-
such as inlet screens-corroding and breaking
apart over time. Proper maintenance procedures
are usually sufficient to prevent this sort
of damage, but when it happens, the results
can be catastrophic.
Corrosion. Anything that enters a turbine,
including fuel and ambient air, has the potential
to react with metal components. Many
types of chemical reactions are possible, and
these can lead to many different types of corrosion.
The effects of corrosion can resemble
both fouling and erosion. Some types of corrosion
cause a buildup of corrosion products,
while others cause metal to be stripped away.
Oxidation is a chemical reaction between
oxygen and metal components. Oxidation rates
increase with temperature, which means it is of
primary concern in areas of high temperature,
such as the combustor or hot sections of blades.
Hot corrosion is a specific type of oxidation that
occurs at high temperatures (>550C) between
metal components and salts deposited on them
(Figure 3). It is of particular concern for gas
turbines because it causes cracking in areas under
extreme stress, such as the bases of turbine
blades. For this reason, maintaining purity of
both fuel and water used for injection is of critical
importance. Contaminants such as sodium
and potassium chloride, sulfur, vanadium, and
lead (the latter two often used as additives in liquid
fuels), can all cause hot corrosion.
Pitting is another form of corrosion that results
in localized formation of holes in metal
surfaces. Pitting occurs when conductive impurities,
such as water, come in contact with miOctober
2016 | POWER
croscopic cracks. Over time, the crack widens
into a pit, which can then lead to much larger
cracks. The same process can occur in crevices
between turbine components. Pitting is most
often a problem in the compressor, where the
blades come into contact with inlet air impurities.
Frequent cycling is also a source of pitting,
as water can condense on compressor blades
after the turbine is shut down.
Fatigue. The extreme temperatures that
turbine blades are subjected to can eventually
result in thermal fatigue. This can be
caused or aggravated by many factors: frequent
cycling; deposits and corrosion that
interfere with heat transfer; combustion and
airflow problems that cause temperature
differentials; excessive vibration; failure to
follow manufacturer recommendations for
controlling thermal stress during startup and
shutdown; as well as others. Fatigue results
in microscopic cracks that can serve as sites
for corrosion as well as growing into large
cracks that result in blade failure.
A more in-depth discussion of degradation
modes can be found in " Gas Turbine Degradation, "
a presentation given at the 2014 Texas
A&M Turbomachinery and Pump Symposia,
from which some of the above is adapted.
Keeping It Clean
The impacts mentioned above can be limited
through the use of anti-corrosion and anti-fouling
coatings and first-stage blades designed for
enhanced durability, but scrupulously maintaining
unit cleanliness over the long haul is
the best approach to prevention. Though some
contaminants can be removed with water washing,
many cannot, and the washing process itwww.powermag.com
ply.
Liquid fuels, because of impurities and
additives, are typically a greater risk than
natural gas, but impurities in gas are also capable
of causing significant erosion and corrosion.
Gary Stansbury, general manager of
gas turbine services for MD&A, told POWER
in an August interview that one of the biggest
sources of problems he sees is dirty gas. " If
dirt and contaminants can get past the strainers
and into the combustor, that will cause a big
issue with the combustion process in the unit. "
Such contaminants in the gas supply can
come from both the pipeline supplying the
plant and the plant fuel supply system, if the
latter was not properly blown out during construction
or at any time it was opened since
then (Figure 4). In addition to cleanliness issues,
dirty gas can change combustion dynamics
and cause increased stress on the turbine,
such as elevated exhaust temperatures. Stansbury
told POWER about a client he worked
with that was experiencing repeated trips as
a result of turbine exhaust temperatures. The
problem was eventually traced to a petroleumbased
product in the gas that the filters were
not properly configured to remove.
Dual-fuel units, particularly those that
operate on fuel oil only as a backup, require
special considerations, Stansbury noted. Liquid
fuel supply systems that sit unused for
months can degrade to the point that they
may not be available on short notice, such
as if the plant has committed to run during
a period when gas is suddenly not available.
" These operators need to understand that
you should run your liquid fuel system periodically
just to make sure all of those components
are working properly, " he said.
For more on these issues, see " Best Practices
for Natural Gas Line Cleaning " in the
49

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POWER October 2016

Table of Contents for the Digital Edition of POWER October 2016